Adding @@session_uuid to vtgate; used as 'context' by Online DDL

[shlomi-noach]

Description

We introduce new session variable:

$ mysql

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> select @@session_uuid;
+--------------------------------------+
| @@session_uuid                       |
+--------------------------------------+
| 29191773-50c1-11eb-8798-f875a4d24e90 |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> select @@session_uuid;
+--------------------------------------+
| @@session_uuid                       |
+--------------------------------------+
| 29191773-50c1-11eb-8798-f875a4d24e90 |
+--------------------------------------+
1 row in set (0.01 sec)

mysql> ^DBye


$ mysql -e "select @@session_uuid"
+--------------------------------------+
| @@session_uuid                       |
+--------------------------------------+
| 3202f39e-50c1-11eb-8798-f875a4d24e90 |
+--------------------------------------+

$ mysql -e "select @@session_uuid"
+--------------------------------------+
| @@session_uuid                       |
+--------------------------------------+
| 32bc2102-50c1-11eb-8798-f875a4d24e90 |
+--------------------------------------+

As can be seen, each session now has a unique session_uuid, accessible to the user. The uuid is generated on session creation, and is unchanged throughout the sessions' life.

The new variable is used by Online DDL functionality. In #7082 we introduced the idea of a migration "context", which is basically some textual identifier. For example, if you run vtctlclient ApplySchema ... with multiple DDLs, they all share the same context, since all run by the same vtctlient invocation.

Now this is the same for vtgate, too. The context includes the session UUID. For example:

mysql> select id, migration_uuid, migration_context from _vt.schema_migrations ;
+----+--------------------------------------+---------------------------------------------+
| id | migration_uuid                       | migration_context                           |
+----+--------------------------------------+---------------------------------------------+
|  1 | 5f2b056d_50c0_11eb_9b88_f875a4d24e90 | vtgate:53137624-50c0-11eb-9b88-f875a4d24e90 |
|  2 | 95bf8bb8_50c0_11eb_9b88_f875a4d24e90 | vtgate:804fd443-50c0-11eb-9b88-f875a4d24e90 |
|  3 | df41df8e_50c1_11eb_8798_f875a4d24e90 | vtgate:dbb6b0f5-50c1-11eb-8798-f875a4d24e90 |
|  4 | df9c28b5_50c1_11eb_8798_f875a4d24e90 | vtgate:dbb6b0f5-50c1-11eb-8798-f875a4d24e90 |
+----+--------------------------------------+---------------------------------------------+

In the above, migrations (3) and (4) were both created by the same vtgate session.

Related Issue(s)

tracking issue #6926

Checklist

• Should this PR be backported? ideally this gets through to v0.9
• Tests were added or are not required
• Documentation was added or is not required

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build

[systay]

LGTM

[shlomi-noach]

fastest review ever!